1. Field of the Invention
The present invention generally relates to fiber optic couplers and, more particularly, to biconical-taper couplers and apparatus for the manufacture of such biconical-taper couplers.
2. Description of the Prior Art
Biconical-taper fiber optic couplers are well known in the art. The performance of a coupler is judged principally by the criteria of uniformity of distribution from the input fiber to the output fibers and the overall loss through the coupler. When measuring the uniformity of a fused biconical taper coupler during fabrication, the limiting factor is the extra output power in the fiber that is injected. The extra power comes from low order modes that are not coupled out of the injection fiber through the down taper. Only high order modes are transferred to the cladding in the minimum taper region. The up-taper redistributes the optical power among all of the fibers in the coupler. The low order modes can be redistributed by deforming the cores to produce a mixing region. A widely used method in use today to produce biconical couplers involves heating them in a region where they are bunched or twisted with the heated region in longitudinal tension. The following U.S. patents disclose various embodiments of such methods for forming biconical fiber optic couplers.
U.S. Pat. No. 4,426,215 to Murphy, discloses twisting fibers together, heating and then tapering to a minimum taper region (MTR). To improve uniformity of the coupler outposts, the MTR is deformed, either by scoring and breaking the MTR, or by pushing the MTR while heating to form a ball. Scoring takes place at the MTR and here the coupler is broken by pulling on opposite sides of the scored MTR. The ends of the two-pieces are then heated and pushed together to permanently rejoin the two regions.
U.S. Pat. No. 4,550,974 to Murphy, discloses low loss fused biconical taper fiber optic coupler that has a biconical taper region where the fibers are fused together. The MTR is deformed. The core material in the deformed region is scrambled with the cladding material as a result of the deformation process so that lower order modes are forced into the claddy.
U.S. Pat. No. 4,291,940 to Kawasaki et al., multimode optic fibers, each having a biconical taper section. The biconical taper sections of the fibers are fused together to provide optical coupling between the fibers. The fused fibers may also be twisted around one another to enhance mode mixing. The access couplers may be produced by fusing two fibers together along a small length, then heating the fused length and pulling the fibers to form the biconical tapers; or by twisting a portion of each of the fibers around one another, applying a tensile force to the twisted portions of the fibers and heating a region of the twisted fibers to soften and fuse a predetermined length of twisted fibers. If the fibers already have biconical taper sections, the access coupler may be produced by twisting the fibers together along their taper sections and heating a region of the taper sections to fuse them together.
U.S. Pat. No. 4,449,781 to Lightstone et al., discloses an optical fiber coupler for operatively coupling together two or more optical fibers. Each coupler consists of several biconically tapered fibers. The biconical sections of each fiber are placed side by side or twisted around one another, and fused together.
As those skilled in the art will appreciate, couplers in which core material is scrambled with the cladding provides an improved output uniformity, yet they are not altogether satisfactory in terms of uniformity of output distribution and low loss.
As also will be appreciated by those skilled in the art, in forming a biconical fiber optic coupler, the insulating jacket (typically a polymer) must be removed from the optic fibers in the region where the coupler is formed. After the coupler is formed, to mechanically support and mechanically stabilize the coupler, a stainless steel strip is epoxyed to the bare fibers and jacketed fibers on either side of the taper region. With prior apparatus for forming biconical-taper couplers, the fibers must be released and reclamped after the coupler is formed in order to secure the stabilizer strip; a time consuming operation.